It is my understanding with the NPPL you can't fly a plane with more than 4 seats (irrespective of the number of POB)
Incorrect, you are restricted by the POB (Max 4 inc Pilot) and weight, not the number of seats.
The medical requirements differ slightly; the NPPL is National and therefore will not be valid to fly EASA aeroplanes beyond 7th April 2015 whilst the LAPL is European and unless you only fly Annex II aeroplanes will become the more predominant licence, valid throughout Europe.
FCL.105 LAPL — Privileges and conditions
(a) General. The privileges of the holder of an LAPL are to act without remuneration as PIC in non-commercial operations on the appropriate aircraft category.
(b) Conditions. Applicants for the LAPL shall have fulfilled the requirements for the relevant aircraft category and, when applicable, for the class or type of aircraft used in the skill test.
FCL.105.A LAPL(A) — Privileges and conditions
(a) The privileges of the holder of an LAPL for aeroplanes are to act as PIC on single-engine piston aeroplanes-land or TMG with a maximum certificated take-off mass of 2 000 kg or less, carrying a maximum of 3 passengers, such that there are never more than 4 persons on board of the aircraft.
(b) Holders of an LAPL(A) shall only carry passengers after they have completed, after the issuance of the licence, 10 hours of flight time as PIC on aeroplanes or TMG.
National Private Pilot’s Licence (Aeroplanes)
Minimum age – 17 years
Privileges and conditions:
(1) Subject to paragraphs (2) to (8) the holder of the licence is entitled to fly as pilot in command of any SSEA, microlight aeroplane or SLMG for which a class rating is included in the licence.
The NPPL requires separate Class ratings the LAPL is all inclusive.
You are unlikely to find anywhere to train for a LAPL in the UK over the next couple of years due to approval costs.